It is well known that polyurethane prepolymers are readily cured with alkali metal salt complexes of 4,4'-methylene dianiline type. The nature of these complexes and their processes of preparation and use are described in such U.S. Pat. Nos. as 3,755,261, 3,876,604 and 4,029,730 and generally may be expressed in the following manner.
The polyurethanes are generally formed by first reacting a polyether polyol or a polyester polyol with a molar excess of an organic diisocyanate to form a prepolymer having terminal isocyanato groups. The prepolymer is then cured to increase its molecular weight from less than about 3,000 to upwards of about 10,000. Examples of such polymers are described in U.S. Pat. Nos. 2,620,516, 2,777,831, 2,843,568, 2,866,774, 2,900,368, 2,929,800, 2,948,691, 2,948,707 and 3,114,735, herein incorporated by reference.
Prior to the curing operation itself, mixing and/or milling of the polyurethane with finely divided particles of the complex is required. Typically the complex particle size is 1 to 150 microns, preferably 2 to 20 microns. Grinding of the complex can be accomplished in standard grinding equipment such as the fluid energy mill and vertical hammer mills employing air classification to remove ground material from the mill.
Mixing of the finely divided complex with liquid prepolymers can be accomplished in dough mixers, high speed impeller mixers, paddle-type mixers and the like. For best results it is preferred that the mix found in the foregoing types of mixers be further mixed (or the complex dispersed) by use of a three-roll mill such as is used in paint and ink manufacture. Improved dispersions can also be prepared in colloid mills. These various types of mixers and mills are described in "The Encyclopedia of Chemical Process Equipment," W. J. Mead, Reinhold (1964).
In the use of a solid type polyurethane prepolymer or gum which is to be cured the complex may be dispersed on a rubber mill or in an internal mixer (a Banbury Mixer). Once the gum has been banded on the mill or broken down in the internal mixer the finely ground complex can be added directly and milling or mixing continued until a uniform dispersion has been prepared.
In mixing the complex with either fluid prepolymers or millable gums the temperature must be kept below the decomposition point of the complex in order to avoid the possibility of premature curing. In the present process the decomposition point of the complex is a function of the particular complex being used and the polymer in which the complex is dispersed. For isocyanato-terminated urethane prepolymers and the sodium chloride complex of methylene dianiline, mixing should be performed below 80.degree. C. and preferably below about 50.degree. C.
The temperature to be utilized during the curing of urethanes will be about 90.degree.-180.degree. C. Temperature is not particularly critical but it must be above the temperature at which the complex dissociates in the system being cured; this temperature will also vary with each particular complex. The complex between sodium chloride and 4,4'-methylenedianiline begins to dissociate at about 90.degree. C. in a urethane system.
Full curing will ordinarily take about 1 minute to 24 hours depending on the temperature for urethanes when utilizing MDA/sodium chloride complex. Preferred curing conditions range from 1/2-12 hours at temperatures ranging from about 100.degree.-150.degree. C. At these preferred curing temperatures sufficient cure to permit demolding occurs within about 5 seconds to 5 minutes because of the high reactivity of the MDA.